1. Field of the Invention
The present invention relates to Analog-to-Digital converters for radio frequency (RF) applications and, more particularly, to a Delta-Sigma modulator capable of reducing quantization noises by digitally correcting errors.
2. Description of the Related Art
Bandpass Delta-Sigma A/D converters perform direct conversion of a RF or IF narrow band signal to digital for processing and heterodyning in digital domain. The ability of performing narrowband conversion at a frequency other than DC makes them particularly attractive for radio applications. Generally, in an input-output stage or a transceiver stage of a certain radio frequency (RF) application field operating at a lower frequency requires a resolution of high precision. Semiconductor bandpass delta-sigma modulators are used to digitize IF signals with high resolution. A signal conversion is performed by making use of a Delta-Sigma A/D converter or a Delta-Sigma D/A converter including a Delta-Sigma modulator and a digital filter. The Delta-Sigma modulator makes use of an oversampling and a noise shaping technology. The oversampling spreads the noise energy over a wider frequency range (FIGS. 1a and 1b). Delta-sigma converters exploit this effect by following the 1-bit ADC with a digital (e.g., lowpass) filter (FIG. 2). This action enables Delta-sigma converters to achieve wide dynamic range from a low-resolution ADC.
FIG. 1a shows how a noise in a signal band can be reduced by using a noise shaping technology, and FIG. 1b shows how a noise in a signal band can be reduced by using an oversampling and a noise shaping technology. The noise shaping technology pushes a quantization noise occurring in a signal conversion to a non-used signal band, and the degree of noise shaping depends upon the order (e.g., 1st, 2nd, 3rd, etc.) of the Delta-Sigma modulator. The oversampling is a technology which reduces the quantization noise within a signal band used by raising the sampling frequency band. By oversampling the traditional pulse counting (Nyquist) frequency of a converter, noise shaping results. Also, the Delta-Sigma modulator operates as a high-pass filter and keeps the signal band high using the oversampling technology, so a noise ratio of the signal band becomes relatively low even though the order of the converter is low and a signal-to-noise ratio (SNR) in the signal band can be improved. Thus, the Delta-Sigma modulation is a technology in which a high resolution can be obtained by reducing the noise within the used signal band by employing noise shaping and oversampling technology.
FIG. 2 shows a block diagram of a conventional A/D converter 200 embodied using a Delta-Sigma modulator. The characteristic of the A/D converter shown in FIG. 2 is determined by the order L of an Analog Loop Filter and the number of bits N of an N-BIT Quantizer in the case where the oversampling ratio OSR is fixed. The heart of a Delta-Sigma modulator and any other analog-to-digital converter (ADC) is a quantizer, a device which maps analog signal amplitude into a finite set of possible representative values, often as few as two (N=1). The quantization operation is inherently nonlinear.
Generally, in the A/D converter 200 embodied using the Delta-Sigma modulator, a 1-bit quantizer (N=1) is used in order to avoid a non-linear characteristic of the N-BIT Quantizer. An A/D converter embodied using the Delta-Sigma modulator with N equal to 1 is also called as a 1-bit A/D converter.
Generally, in order to obtain an appropriate characteristic of a signal-to-noise ratio (SNR) required in each application field, an inverse proportional relation is established between the order L and the OCR of the modulator. For example, in the case of a voice band (4 kHz, Fs=8 kHz), it is desired that L=2, OSR=256 and N=1, while in the case of an audio band (20 kHz, Fs=44.1 kHz), it is desirable that L=4, OSR=64 and N=1. Here, Fs is a sampling frequency. According to Nyquist theory, Fs must be at least the Nyquist Frequency FN which is twice the bandwidth of the input signal. The oversampling ratio OSR is the ratio of Fs to the Nyquist frequency FN A deterioration of a signal-to-noise ratio (SNR) caused by a low L value can be canceled by increasing OSR.
Since the oversampling operation is performed in a sampling frequency Fs that is higher than the Nyquist frequency by the OSR, it is often not practicable to construct a circuit using the oversampling technology in applications involving higher frequency RF signal bands (e.g., microwave bands). If the OSR is 256 and the Nyquist frequency FN is 2 MHz, an operating frequency becomes 512 MHz, which presents a challenge. And generally, if the OSR is lowered, the order L of the modulator is increased, so that there are many disadvantages in noise, power consumption and cost when a circuit uses a lowered OSR. Particularly, in case of an analog modulator, since most of circuits include analog circuits which are sensitive to noise, the modulator is significantly influenced by the noise and it is not easy to implement it.
Recently, a variety of methods have been suggested in which even using a lowered OSR and a lowered order L of a modulator, an appropriate noise characteristic for any RF application can be obtained. Some major methods among them include a multi-stage noise shaping (MASH) method wherein a quantization noise is added in each stage and the noise is removed by filtering Another method is to use a dual quantizer of a 1-bit and an N-bit, removing the quantization noise occurred in the 1-bit quantizer in an analog method, and outputting through a digital noise shaping method. The method of using the dual quantizer of the 1-bit and the N-bit, and removing the quantization noise through digital noise shaping is disclosed in U.S. Pat. No. 6,300,890 and shown in FIG. 3.
FIG. 3 shows a conventional Delta-Sigma modulator 300 that can quantize an analog input signal X using a first feedback loop including the 1-bit quantizer 3 (Q1), and reduce a quantization noise by performing a quantization noise shaping using a second feedback loop including a multi-bit quantizer 11. The Delta-Sigma modulator shown in FIG. 3 subtracts a quantized analog output signal of a 1-bit D/A converter 4 from an analog output signal of a final stage of integrator 8 in order to calculate a quantization error, and then removes the quantization noise E1 introduced in the 1-bit D/A converter 4. The integrator acts as a lowpass filter to the input signal and a highpass filter to the quantization noise. Thus, most of the quantization noise is pushed into higher frequencies (FIG. 1b). Oversampling has changed not the total noise power, but its distribution. However, since a signal subtracted in a subtractor 10 is an analog signal and a signal loop for removing the quantization noise is an analog circuit, the modulator can be much affected by noise.